The best way to hold your own in a debate as complex as the one about human nature is to understand what philosophers, scientists, psychologists and others have already contributed to it. Typically, "human nature" refers to the set of traits that human beings are born with. This means that anything which people learn, including culture, is not part of their nature. If it were part of their nature, they would not need to learn it.
The idea that human nature is good stretches back to Plato and Aristotle, who argued that human nature is social and people tend to get along with each other. Science supports this viewpoints with studies that show that young children empathize with others who are in distress, suggesting that they were born with a concern for others.
St. Augustine argued the opposite, that human nature is sinful, violent and greedy. Scientists like Richard Dawkins seem to agree with him, arguing that humans evolved to be selfish because this is the only way that natural selection could have worked.
The debate about human nature is more than a question of good versus evil, though. Sigmund Freud argued that human nature is a conflict between both good and evil rather than simply one or the other, and that there is scientific research to support both sides of the argument. John Locke and Jean-Paul Sartre doubted that human nature exists at all, though Locke believed that humans were shaped by their environment while Sartre believed that humans were shaped by their own free decisions. Finally, moral relativists argue that no human trait is inherently good or evil and that their values depend on culture and situation.